ICode9

\[设等比数列a_{n}=ar^{n-1},首项为a_{1},r为公比,n\in N^{*}.\\ 求其前n项之和(设为s_{n}) \]\[\\ \\ \]\[s_{n}=a_{1}+a_{2}+a_{3}+...+a_{n}=a_{1}r^0+a_{1}r^{1}+a_{1}r^{2}+...+a_{1}r^{n-1} \]\[\\ \\ \]\[设s_{u}=r \cdot s_{n} \\ =r(a_{1}r^{0}+a_{1}r^{1}+a_{1}